Belgrade, capital city of Serbia has a population approaching 2,000,000, generating approximately 35 to 40% of the GDP of Serbia. The city is located at the confluence of the rivers Danube and Sava. The old city of Belgrade was founded on the south bank on an easily defendable outcrop of limestone. The continuing expansion of the city northwards has been significantly constrained by the limited capacity of the existing bridges crossing the river Sava, creating a particular bottle neck increasing the level of traffic congestion in the centre of Belgrade. In order to reduce traffic congestion in the City of Belgrade and increase capacity on the network a third major road bridge across the Sava River being part of the Inner City Semi-Ring Road is under construction, situated 4 kilometres upstream from its confluence with the river Danube. The structure, including the approaches shall be the biggest viaduct of the Balkan region, connecting New Belgrade on the north bank with Radnicka street on the south of the river.

The new bridge has been designed as a landmark structure for the people of Serbia incorporating a needle shaped Pylon with a height of 200m. The steel Main 376m Span of the bridge is supported by 80 stay cables anchored at the Pylon structure and is counter-balanced by a post-tensioned, reinforced concrete back span of 200m. The approach towards New Belgrade is constructed as a 388m post-tensioned, reinforced concrete side span as continuous beam box girder with a similar arrangement of deck as the back span.

The deck has been designed to accommodate six traffic lanes with shoulders and a double track light rail line, in addition to two paths for the use of pedestrians and cyclists, resulting in and overall width of 45m. In comparison, a standard 6 lane highway bridge would generally have a deck width of approximately 28m to 29m. The overall length of the continuous bridge deck comprising 7 spans is 964m. The deck for the Main Span has been designed as a structural steel box girder of 4.75m overall height assembled by the free cantilever method crossing the river Sava 20m above water level. The decks of Back Span and Side Spans are composed of pre-stressed concrete and installed using the incremental launching method.

The 7 piers are founded on deep bored piles of 1.5m diameter, approximately 30m long. The Pylon is supported by a foundation designed as a composite “diaphragm wall-deep pile” footing, the diaphragm wall creating a cylinder 36m diameter and 37m deep with a central core consisting of a grid consisting of 113 cast concrete piles of similar depth. This design was developed in order to achieve the optimum economy whilst ensuring the stability of the supported structure.

The scale, complexity and advanced technology of the project structures, including the height of the pylon, length of the main span, width of deck and elevated casting yards as well as the limited construction time of 3 years are challenging conditions in international environment created by the participation of specialist companies and engineers from Serbia, Austria, Germany, Slovenia, China, Switzerland, France, United Kingdom, Denmark and Hungary.

The Preliminary Design was completed by Ponting Maribor, DDC Ljubljana & CPV Novi Sad in 2006, following being awarded the first place in an international competition held in 2004.
When completed, the new Sava Bridge will be the largest single pylon cable-stayed bridge in Europe. The load of the 376m long Main Span is transferred through 40 twin cables to the pylon and continue to the 200m long Back Span. The longer Main Span is constructed of structural steel in order to keep the weight of the structure down to a minimum possible. This is in turn balanced by the shorter but heavier Back Span constructed in concrete. On the south side one end span extends the bridge to the connection to the approach ramps. On the north side the Side Span consists of 4 spans of 70, 108, 80, 80m. Construction is similar to that of the Back Span, being post-tensioned, reinforced concrete. Fixing point for the deck is the pier table at the pylon.

Figure 1: General Layout

The outer dimensions of the superstructure are carried through the entire length of the bridge despite the use of two different construction materials. The deck slab is supported by a hollow box girder 14.50m wide and 4.75m high. The outer 15.25m wide cantilever slabs are supported every 4m by inclined struts. To distribute the stay cable forces a 3 cell hollow box is used. This 45m wide deck will carry six traffic lanes and a double track light rail line, as well as two paths for the use of pedestrians and cyclists.

Figure 2: Cross Section of Back Span

Seismic loads and wind loads have been determined considering local conditions. Wind tunnel tests for aerodynamic studies were performed. The entire design follows Euro codes. As a requirement the unlikely loss of a strut and a cable is considered in the design which requires a 100 year design life of the bridge in service.

The bridge is being constructed using a Design-Build approach under FIDIC Conditions of Contract for Plant and Design Build, (First Edition, 1999), this form of contract being considered as most appropriate for capital infrastructure projects with international participants.

The design and construction contract has been awarded in April 2008 to the joint venture Ogranak Sava Most, an international consortium consisting of PORR, DSD und SCT. PORR has taken the role of technical and commercial coordinator of the consortium, being responsible for the construction of all piled foundation works, substructure and superstructure of the Back Span, Pylon and installation of the bridge stay cables. DSD is responsible for the supply and erection of the structural steel for the Main Span. SCT is responsible for construction of the Side Span together with the supply of reinforcing steel for the Pylon concrete and all ancillary works. The lead contractors are in turn supported by numerous Serbian construction companies supplying the major part of the workforce, ready-mixed concrete and other construction materials.

The detailed design of the substructures and superstructures of the bridge and side span is being prepared by Leonhardt, Andrä and Partners, Consulting Engineers of Stuttgart, Germany, a design consultant with extensive experience in the design of major stay cabled bridges. The design of foundations has been the responsibility of the geotechnical division of PORR.

The design and construction project is being administered by international consulting engineers, Louis Berger Group in association with Eurogardi Group, consulting engineers, Novi Sad, Serbia performing the role of the Engineer and supported by the Institute Kirilo Savic, Belgrade who are performing the technical supervision of the works. The design is assessed by the project Engineer with the support of a committee of Technical Experts led by prof. Nikola Hajdin, President of Serbian Academy of Sciences and Arts, appointed by the City of Belgrade through Belgrade Land Development Public Agency, the Employer. The Faculty of Civil Engineering of Belgrade University formed a separate committee to perform the design technical control, each member of this committee being a professor specialist in specific aspects of bridge design and construction.

Construction Sequence

The site works for the bridge began on April 14th 2009, following a period of extensive geotechnical investigations of sub-soil conditions and testing and approvals of materials to be used in the structures, simultaneously the preparation and approval of the design in coordination with the Contractor’s Programme of Works.
A stipulation for the construction of the bridge is that the main span across the Sava River is achieved without using temporary supports located in the river. This constraint has lead to the adoption of free cantilever method supporting the deck with the stay cables in order to comply with this requirement, thus enabling an unrestricted passage for shipping. The critical path of construction activities is given by those milestones which affect the start of the free cantilever. This requires that the pylon needs to reach a minimum height of 130m and the Back Span must be connected to the pylon through the pier table together with the completion of the construction of the composite transition segment of the deck that creates the interface between the different forms of deck construction.

Figure 3: Construction sequence

The Side Span shall be constructed independently of the main bridge and simultaneously with the Main Span. Following the completion of the link between the 2 structures at axis 5, pavement and finishing works can be completed.

The geological profile through the bridge site generally consists of river sediments in the upper strata, with marl, and limestone towards south bank and to the north bank deposits of gravels beneath the sediments and sand. These sub-soil conditions determined that all piers and auxiliary columns are founded on bored piles with diameters 1.20 to 1.50m to a depth between 30m and 37m. To confirm the soil values with shaft friction and base resistance four trial piles were executed and tested to the double working load. The pylon is founded on a combined circular diaphragm wall with 113 bored piles to a depth of 37m connected by a massive base plate, generally termed a “pot” foundation. Special attention during construction was given to the connection of the base plate to the diaphragm wall which transfers 50% of the vertical reaction forces from the pylon. In excess of 16.000m³ of concrete was used in the construction of the pylon foundation.

The post tensioned concrete structure of the back span crosses Cukaricki bay, a former side arm of the River Sava. It is constructed on an elevated casting yard in a height of approximately 20m behind pier 7 in segments with length of 18m. From there it is launched towards the pylon using 3 temporary braced piers in a distance of 50m. Due to the exceptional size of the deck the fabrication area was divided in 3 sections. In the rear part the bottom plate is installed. In the middle part the 4.75m high triple hollow box with the installed pre-casted stay cable anchorages is poured. The cross-section of the superstructure consists of a 45m wide transverse post tensioned cantilever deck with a central 3 celled concrete box girder with the outer cantilevers supported by cast in steel struts. The arrangement of segments on the launching platform allows construction work to proceed simultaneously in three sections of deck segments in order to reduce construction time.

The weight of this 200m long structure during final launching operation will be approximately 20,000 tonnes. To move this weight two sets of hydraulic lift-push equipment located in axis 7 underneath the four webs are used. Bridge closure will be done on the scaffolding at the pier table to achieve the required connection to the pylon and further structure. The auxiliary piers shall be removed on completion of the structure of the Main Span and installation of the supporting stay cables.

The new significant landmark for Belgrade will be a tall Pylon with a height of 200m especially developed in order to enhance the elegance of the viaduct. The pylon is composed of conical shaped form made of concrete. In the lower part it is separated into two legs which penetrate the deck between street lanes and light rail track. The two legs merge in a height of 98m to continue in a single circular shaft bearing the stays. The formwork is designed to deal with the continuous changing radius of the cone of 16m at the foundation base plate, reducing to 4m radius at the height of 175m. The upper 25m of the pylon shall be constructed in structural steel with a stainless steel cladding.

One pylon leg is equipped with a service elevator located in a shaft; the other leg has a shaft containing emergency stairs.

Construction of the pylon is achieved with self climbing formwork in 5 sections of 4.39m up to the bridge deck level, continuing with 34 sections of 4.59m up to a height of 175m. To reduce construction time the two legs are raised through the pier table section following the cross-section of the pylon. The pier table is built on heavy falsework and connected to the pylon legs by reinforcing couplers and post-tensioning tendons. The separate sections of formwork merge at the elevation of 98m. The separate inclined legs are supported against each other by 3 compression struts until the sections merge.

Figure 8: Self climbing formwork for pylon

The upper 25m of the pylon consists of a stainless steel tip supported by a construction of structural steel. The pylon finishes at a height of 200m with a diameter of 1.50 m. The cone end has no structural function but provides the pylon an elegant and tall appearance. It is also Belgrade´s biggest conductor and therefore well protected from lightning strikes by appropriate earthing.

The main span superstructure is to be constructed with a total of 8,600 tons of high quality bridge construction steel grade S355J2+N. Component parts of the deck are being manufactured in China in advance and delivered in transportable units of maximum length 17.156mm, maximum height 2575mm, maximum width 4148mm and maximum weight 41.4t on a combined sea and river-route via Rotterdam through the Rhine-Main-Danube to the preassembly yard beside the construction site at Mala Ciganlija in Belgrade constructed by DSD.

The delivered parts are assembled by welding into sections using a 48t capacity gantry crane. Corrosion protection consisting of primer and intermediate coats is completed in the workshop, weld seams protection is completed after welding at the pre-assembling yard. Special heavy load transport devices will be used to transport the pre-assembled sections onto pontoons to transport them 500m upstream the Sava to the place of lifting. A derrick crane shall be used to lift up the four sections which form a segment with self weights between 374t up to 208t. The section length of segments has been determined by the stay cable anchorage distance of 16m. After lifting, adjusting and welding to the cantilevered deck the weight of the new deck section is transferred to the Back Span via the pylon by two pairs of stays before continuing with the next section within a 28 days cycle for each of the 19 Segments M2 to M20.

Figure 10: Installed transition element T1

The first two segments T1 (500t) and M1 (440t) in the area of the platform adjacent to the pylon being the heaviest parts require a different methodology for erection. Heavy load temporary supports independent from the concrete pylon support T1 which is assembled immediately beside the pylon and then lifted by mobile cranes. Final transition of reaction forces from steel Main Span to concrete pier table is achieved by overlapping post-tensioned tendons and stud bolts. Next segment, M1, shall be transported similarly to T1 by trucks and pontoons from Mala Ciganlija and erected using the derrick crane already installed onto T1.

The stay cables used to support the bridge deck shall have a maximum length of approximately 373m for the outer cable to the Main Span. The stay cables are formed from compacted bundles of parallel 7-wired stranded steel cables. Each individual strand is galvanized and sheathed with polyethylene coating filled with grease in order to prevent corrosion. The outer diameter of the covering HDPE pipes finished in a silver grey colour will vary from 200 to 280mm depending on the number of strands enclosed. In total 1280 tons of high grade steel is used for the 80 cables with up to 91 strands. A strand by strand installation method will be employed. The upper anchor heads transfer the cable loads directly into the pylon via bearing plates. Ring nuts screwed onto the lower anchor heads of the deck anchorages allow later adjustment of cable forces.

After the completion of the launching of the Back Span the entire hydraulic equipment together with the steel launching nose will be moved to the launching platform constructed for the Side Span. The casting yard is positioned within the first span between piers 1 and 2. To reduce the span during 36 stages of launching operation temporary piers are placed between the permanent piers at 40m spacing. The bridge deck is designed with a single cell hollow box girder with primary centric post tensioned tendons for launching and secondary draped tendons within the web to achieve final spans of 70m, 108m, 80m and 80m. They are stressed after launching in order to allow the removal of the temporary piers. A special challenge is the required camber for the big span. This requires construction and launching on specially designed camber beams, developed for this purpose.

In order to push forward the heavy superstructure with a weight of in excess of 30.000t, two twin-sets of hydraulic lift push equipments developed and designed by SCT will be used. These devices are assembled behind each other on top of pier 2. The horizontal reaction force during launching is distributed by concrete struts into the foundation of the casting yard.

On reaching pier 5 the steel nose will be dismantled and a steel connection element T2 will be attached. During this operation the last span is poured within the casting yard with an extended cantilever slab to connect the approach roads. To close the bridge the finished Deck of Side Span is pushed for 10 cm towards the cantilever of the Main Span using the pre-adjusted final bearings.

After closure of the gap between Main Span and Side Span ancillary works such as water proofing, concrete curbs, road asphalt, safety barriers, gravel base for LRT lane, handrails, street and river lamps, signboards and public utilities shall be installed prior to the final adjustment of all stay cables, due to the weight of these finishing works approaching the self weight of the structural steel of the Main Span.

Comparison of the new Sava bridge with other world famous bridges illustrates the magnitude of the project undertaken.

The most common criteria used for ranking cable-stayed bridges is the length of the main span. The Sutong bridge in China and the Stonecutter’s bridge in Hong Kong are the cable-stayed bridges with the longest spans in the world (1088m and 1018m respectively). With a main span of 376m, the bridge over river Sava shall rank in the top 10 list of long span cable-stayed bridges constructed in Europe.

However, in the category of SINGLE-pylon cable-stayed bridges, with its 376m main span and deck width of 45m together with the 200m pylon, Sava bridge is the largest asymmetric cable-stayed bridge being constructed in the world at this present time, with almost 16.935m2 of main span hanging on ONE pylon.

Such outstanding features outperform the so far many of the existing stay cabled bridges constructed to date.